Most of the matter that one can observe in the universe is in a plasma state comprised of electrically charged particles, electrons and ions. It is therefore imperative to understand the behavior of plasmas in order to explain galactic formation, gamma-ray bursts, and our own Sun. Often, the temperature of electrons in a plasma is hotter in one direction than in the other. Such plasmas are said to be anisotropic. An anisotropic plasma eventually becomes isotropic -- reaching the same temperature in all three dimensions of space -- by a process known as the Weibel instability. The Weibel instability was predicted by Weibel almost 50 years ago, but it has been impossible to check the theory of how rapidly this instability grows and why it eventually stops growing (saturates) because it has not been possible to generate a plasma with known temperature anisotropy in the laboratory. This project aims to measure the generation and saturation of the Weibel instability for the first time. This experimental work will be performed by the University of California - Los Angeles team at the Accelerator Test Facility at Brookhaven National Laboratory, and will involve training of a graduate student and a postdoctoral associate in plasma and accelerator science.
Highly anisotropic plasmas can be produced by optical field ionization of He gas using a circularly polarized laser pulse, resulting in a plasma that is very hot in the transverse direction but rather cold in the direction of propagation of the laser. Such a plasma undergoes a hierarchy of plasma kinetic instabilities, starting with the two stream instability and current filamentation instability that reduce the plasma anisotropy from ~100 to less than 10 in a pico-second. Thereafter, the electron Weibel instability begins to grow, seeded by coalescence of currents associated with the filamentation instability. The pure Weibel instability is characterized by rapid growth of a static magnetic field with a broad wavenumber spectrum. This wavenumber spectrum quickly narrows to a fairly well defined, most unstable mode that has helicoid topology. This work will measure the exponential growth and saturation of this magnetic field and its topology for the first time. To measure the dynamics of the Weibel B-field, a relativistic electron beam will be used as a probe. During its passage through the plasma, the probe electrons will be deflected by the transverse component of the helicoid magnetic field. These deflections can be visualized as probe electron density structures on a screen placed some distance away from the plasma. From images taken at different times, the evolution of the k spectrum, its spectral amplitude and 2D spatial profile of the B-field can be obtained, thereby allowing a quantitative comparison of experiment with kinetic theory of the Weibel instability.
This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.
